This invention relates to a decoder for receiving, decoding and conversion of frequency domain encoded signals, e.g. MPEG-2 encoded video signals, into standard output video signals, and more specifically to a decoder which converts and formats an encoded high resolution video signal to a decoded lower resolution output video signal.
In the United States a standard, the Advanced Television System Committee (ATSC) standard defines digital encoding of high definition television (HDTV) signals. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The standard is described in an International Standard (IS) publication entitled, xe2x80x9cInformation Technologyxe2x80x94Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626xe2x80x9d, ISO/IEC 13818-2, IS, 11/94 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard.
The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level is intended for coding video signals conforming to existing television standards (i.e., NTSC and PAL). Another standard, known as Main Profile, High Level is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line.
The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 720*567*30 or 12,247,200 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152*1,920*30 or 66,355,200 pixels per second. This data rate is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this standard is still far greater than the maximum data rate for the Main Profile, Main Level standard.
The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats to be covered by the standard. These formats define images having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG-2 bit stream representing a sequence of images in five layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, and the macroblock layer. Each of these layers is introduced with control information. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zig-zag patterns and dequantization information) are interspersed throughout the coded bit stream.
Format conversion of encoded high resolution Main Profile, High Level pictures to lower resolution Main Profile, High Level pictures; Main Profile, Main Level pictures, or other lower resolution picture formats, has gained increased importance for a) providing a single decoder for use with multiple existing video formats, b) providing an interface between Main Profile, high level signals and personal computer monitors or existing consumer television receivers, and c) reducing implementation costs of HDTV. For example, conversion allows replacement of expensive high definition monitors used with Main Profile, High Level encoded pictures with inexpensive existing monitors which have a lower picture resolution to support, for example, Main Profile, Main Level encoded pictures, such as NTSC or 525 progressive monitors. One aspect, down conversion, converts a high definition input picture into lower resolution picture for display on the lower resolution monitor.
To effectively receive the digital images, a decoder should process the video signal information rapidly. To be optimally effective, the decoding systems should be relatively inexpensive and yet have sufficient power to decode these digital signals in real time. Consequently, a decoder which supports conversion into multiple low resolution formats must minimize processor memory.
The present invention is embodied in a digital video signal processing system which receives, decodes and displays video signals that have been encoded in a plurality of different formats. The system includes a digital video decoder which may be controlled to decode the encoded video signal and, optionally, provide a reduced resolution version of the decoded video signal. The system processes the received encoded video signal to determine the format and resolution of the image which would be produced if the signal were decoded. The system includes a controller which receives the determined format and resolution information and which also receives information concerning the format and resolution of a display device on which the received image will be displayed. The controller then generates signals to cause the digital video decoder to provide an analog video signal having a resolution and aspect ratio that is appropriate for the display device.
According to one aspect of the invention, the encoded video signals are encoded using a frequency-domain transform operation and the digital video decoder includes a low-pass filter which operates on the frequency-domain transformed digital video signal.
According to another aspect of the invention, digital video decoder is coupled to a programmable spatial filter which is responsive to a control signal provided by the controller to resample the decoded digital video signal provided by the digital video decoder to produce a digital video signal which conforms to the aspect ratio and resolution of the display device.
According to another aspect of the invention, the digital video signal is encoded according to using an encoding technique specified by the moving pictures experts group (MPEG) and the aspect ratio and resolution of the encoded video signal are extracted from the header of a packetized elementary stream (PES) packet received by the digital video decoder.
According to another aspect of the invention, the digital video signal is encoded according to using an encoding technique specified by the moving pictures experts group (MPEG) and the aspect ratio and resolution of the encoded video signal are extracted from a sequence header of a video bit-stream received by the digital video decoder.
According to another aspect of the invention, the system includes a user input device through which a user may configure the system to produce an output video signal which is compatible with the display device.
According to another aspect of the invention, the system includes apparatus which automatically determines the aspect ratio and resolution of the display device.
According to another aspect of the invention, the system includes apparatus which sequentially produces video signals corresponding to a plurality of display device types and is responsive to a selection signal provided by a user to identify one of the display types as corresponding in resolution and aspect ratio to the display device.